Controlled geometry hardmask including subresolution elements

ABSTRACT

Methods for forming accurate, symmetric cross-section spacers of hardmask material on a substrate such as a silicon wafer or quartz substrate, for formation of precise subresolution features useful for forming integrated circuits. The resulting symmetrical hardmask spacers with their symmetric upper portions may be used to accurately etch well-defined, high aspect ratio features in the underlying substrate. Some disclosed methods also enable simultaneous formation of hardmask structures of various dimensions, of both conventional and subresolution size, to enable etching structural features of different sizes in the underlying substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 10/883,215,filed Jul. 1, 2004, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the fabrication of integrated circuits.More specifically, the present invention relates to a method ofproducing fine geometry hardmasks with a controlled profile and heightto enable the fabrication of precise subresolution features forintegrated circuits and other structures.

2. State of the Art

In fabrication of semiconductor devices including integrated circuitry,such as memory dice, conventional photolithography limits the ability toproduce very fine structural features. Currently, photolithography isonly capable of producing structural features of about 100 nm in minimumdimension. This inadequacy limits the ability of a manufacturer toproduce extremely small structural features for integrated circuitsthrough conventional photolithography processes. A capability to furtherreduce the dimensions of structural feature size is particularlyimportant to the fabrication of semiconductor memory devices to enableincreasing the number of memory cells on such semiconductor memorydevices of a given size.

U.S. Pat. No. 6,514,849 to Hui et al., U.S. Pat. No. 6,537,866 toShields et al., U.S. Pat. No. 6,455,433 to Chang et al., U.S. Pat. No.6,110,837 to Linliu et al., U.S. Pat. No. 5,916,821 to Kerber, U.S. Pat.No. 5,776,836 to Sandhu, and U.S. Pat. No. 5,296,410 to Yang attempt toovercome some of the problems associated with conventionalphotolithography.

An alternative to using conventional photolithography is a techniquecalled “loose photo patterning.” Generally described, loose photopatterning allows creating smaller mask features than would be possiblewith conventional photolithography. In loose photo patterning, maskfeatures of conventional size are formed using conventionalphotolithography and dry etching, followed by coating such features witha layer of material. The layer of material is then removed from the topof the mask feature and the mask feature is subsequently etched away.These side from the top of the mask feature and the mask feature issubsequently etched away. These side coatings can be used as a hardmaskto form so-called “subresolution” structural features, indicating thatsuch structural features are of smaller dimensions than are achievableby using photolithography to form them directly. In other words, suchfeatures are smaller than the finest resolution photolithographyprocesses can produce. Furthermore, all subresolution features will bethe thickness of the coating used to coat the standard mask feature.Loose photo patterning allows creating mask features as small as 10 nm,which would not be possible with conventional photolithography.

A method of forming features using conventional loose photo patterningwill be better understood with reference to FIGS. 1A-1D. FIG. 1A showsportion 100 of substrate 2, such as p or n type silicon or othersemiconductor substrate material, including a first layer 4 deposited onsubstrate 2. First layer 4 is typically a silicon nitride layerapproximately 900 Å in thickness. First layer 4 is formed into aselected geometry, as shown in FIG. 1A, using conventionalphotolithography and anisotropic etch processing. Referring to FIG. 1B,a hardmask layer 6 of, for example, 300 Å thick tetraethyloxysilicate(TEOS) silicon dioxide is deposited on first layer 4. As shown in FIG.1C, hardmask layer 6 is anisotropically etched to leave only the portionof hardmask layer 6 covering the sidewalls of first layer 4. First layer4 is then completely removed from substrate 2 by a dry or wet etch toform the sidewall spacer hardmask shown in FIG. 1D usable for furtheretching of substrate 2 to define selected structural feature patternstherein.

While conventional loose photo patterning allows for forming finegeometry hardmask features, it also results in a phenomenon known as“sputtering.” As shown in FIG. 1D, sputtering occurs when hardmask layer6 exhibits an asymmetric profile, which results in a poorly definedprofile in the etched features of underlying substrate 2. Theseasymmetries of hardmask layer 6 produce different etch rates adjacentthe inner and outer edges of hardmask layer 6 when the underlyingsubstrate 2 is etched. As the aspect (height or depth to width) ratiosof etched features in substrate 2 increase, the phenomenon of sputteringis aggravated and it becomes more important for the profile of hardmasklayer 6 to be symmetric and, preferably, rectangular.

Therefore, due to the limits of conventional photolithography and loosephoto patterning it is desirable to develop a method which results inhardmask elements with an accurately controlled profile and height,enabling the semiconductor device fabricator to achieve an accuratelyetched profile in a substrate underlying the hardmask.

BRIEF SUMMARY OF THE INVENTION

The present invention, in a number of embodiments, includes methods forforming accurate, symmetric cross-section hardmask elements on anunderlying substrate to enable the fabrication by etching of precisestructural features in the substrate and resulting end productsincorporating such features. The resulting hardmask elements may be usedto accurately etch well-defined, high aspect ratio features in thesubstrate free of sputtering defects.

In one exemplary embodiment, a method for forming hardmasks on asubstrate is provided. First, a substrate is provided. Next, a firstlayer of a material may be formed on the substrate. Following formationof the first layer, a second layer may be formed on the exposed surfaceof the first layer. Next, a plurality of discrete structures may beformed on the substrate by masking, patterning and etching the first andsecond layers to a geometry defined by an exposed surface of the secondlayer and substantially vertical sidewalls extending upward from thesubstrate to the exposed surface. Following formation of the pluralityof structures, a hardmask layer made may be deposited over the substrateand the plurality of discrete structures. Portions of the hardmask layeradhered to the exposed surface of the second layer may then be removedby an etching process, which also removes the portions of the hardmasklayer on the substrate between the discrete structures, while theportions of the hardmask layer flanking the discrete structures remainas spacers, exhibiting an asymmetric profile. The discrete structuresmay then be planarized by an abrasive process such as chemicalmechanical planarization (CMP) to remove the entire second layer and thelaterally adjacent, uppermost, asymmetric ends of the remaining hardmaskportion spacers. Following planarization, the first layer may be removedby a selective etching process, leaving only the remaining spacersformed as portions of the sidewalls of the original hardmask layer,providing well-defined, symmetrical hardmask elements for etching of theunderlying substrate. The method of the above exemplary embodimentproduces hardmask features having a well-defined, symmetriccross-section.

In another exemplary embodiment, a method for forming hardmask elementsof various sizes on a substrate is disclosed. First, a substrate isprovided. A first layer of a material may then be formed on thesubstrate. Following formation of the first layer, a second layer may bedeposited on the exposed surface of the first layer. A plurality ofdiscrete structures may then be formed by masking, patterning andetching the first and second layers to a geometry defined by an exposedsurface of the second layer and substantially vertical sidewallsextending from the substrate to the exposed surface. Following formationof the plurality of structures, a hardmask layer may be deposited overthe substrate and the plurality of discrete structures. The portion ofthe hardmask layer adhered to the exposed surface of the second layermay then be removed by an etching process, which also removes theportions of the hardmask layer between the discrete structures, whileportions of the hardmask layer flanking the structures remain as spacerswhich exhibit an asymmetric profile. The discrete structures may then beplanarized by an abrasive process such as CMP to remove the entiresecond layer and the laterally adjacent, uppermost ends of the remaininghardmask portion spacers. Following planarization, the exposed uppersurface of the first layer of material of at least one of the structuresmay be protected and the unprotected portions of the first layer of thestructures removed by an etching process. The method of the aboveexemplary embodiment produces hardmask features of various sizes havinga well-defined, symmetric cross-section.

In yet another exemplary embodiment, a method for forming hardmasks ofvarious sizes on a substrate is disclosed. A substrate is provided and afirst layer of a material may be formed on the substrate. Followingformation of the first layer, a second layer may be deposited on theexposed surface of the first layer. After formation of the second layer,a third layer of a material may be formed on the exposed surface of thesecond layer. A plurality of discrete structures may then be formed bymasking, patterning and etching the first layer, the second layer, andthe third layer to a geometry defined by an exposed surface of the thirdlayer and substantially vertical sidewalls extending from the substrateto the exposed surface. After formation of the plurality of discretestructures, the exposed upper surface of at least one of the pluralityof discrete structures may be protected. A hardmask layer may bedeposited over the substrate and the plurality of discrete structures.Following deposition of the hardmask layer, the portions of the hardmasklayer on the substrate and on the upper surfaces of the discretestructures may be removed by an etching process, the etching alsoremoving the third layer and a portion of the second layer on anyunprotected discrete structure. The plurality of discrete structures maybe planarized to remove the second layer, the planarization stopping onthe first layer of the unprotected discrete structures and the thirdlayer of the at least one protected discrete structure. Finally, theexposed portions of the first layer may be removed by an etchingprocess. The method of the above exemplary embodiment produces hardmaskfeatures of various sizes having a well-defined, symmetriccross-section.

These features, advantages, and alternative aspects of the presentinvention will be apparent to those skilled in the art from aconsideration of the following detailed description taken in combinationwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which illustrate what is currently considered to be thebest mode for carrying out the invention:

FIGS. 1A-1D are schematic sectional views illustrating a conventionalloose photo patterning process.

FIGS. 2A-2I are schematic sectional views illustrating an exemplaryembodiment of a method of the present invention.

FIGS. 3A-3D are schematic sectional views illustrating another exemplaryembodiment of a method of the present invention.

FIGS. 4A-4K are schematic sectional views illustrating yet anotherexemplary embodiment of a method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the description which follows, like features and elements in theseveral embodiments are identified with the same or similar referencenumerals for the convenience of the reader.

FIGS. 2A-2I illustrate an exemplary embodiment of a method of thepresent invention suitable for producing hardmask elements having anaccurate, symmetric, rectangular cross-section. Referring to FIG. 2A,substrate 2 is provided made from a suitable semiconductor substratesuch as silicon, polysilicon, or a layered semiconductor structure suchas a silicon on insulator (SOD structure, as exemplified by silicon onglass (SOG) and silicon on sapphire (SOS) structures. Substrate 2 mayalso be a glass material useful for forming reticles, such as soda-limeglass, borosilicate glass, or quartz. Then, as shown in FIG. 2B, a firstlayer 4 having upper surface 16 may be formed on and adhered to portion200 of substrate 2 using techniques such as, for example, chemical vapordeposition (CVD), physical vapor deposition (PVD), or atomic layerdeposition (ALD). First layer 4 may be formed from materials such assilicon nitride. In this exemplary embodiment, first layer 4 may beformed from 500 Å thick silicon nitride.

Referring to FIG. 2C, following deposition of first layer 4, secondlayer 8 may be deposited on top of and adhered to the upper surface 16of first layer 4. Second layer 8 may be formed from TEOS-type silicondioxide. Second layer 8 may be deposited using techniques such as, forexample, CVD, PVD, or ALD. In this exemplary embodiment, second layer 8may be formed from 500 Å thick TEOS-type silicon dioxide.

Referring to FIG. 2D, following deposition of second layer 8, portion200 may be covered with a patterned and developed photoresist 12,etched, and discrete structures in the form of regions 210 formed frometched first and second layers 4, 8 using conventional photolithographytechniques. Referring to FIG. 2E, regions 210 are shown after etchingand subsequent removal of photoresist 12. As shown in FIG. 2E, regions210 formed of the remaining segments of first layer 4 and second layer 8may exhibit a geometry defined by substantially vertical sidewalls 24extending from substrate 2 to exposed upper surface 26 of second layer8.

Referring to FIG. 2F, following forming of regions 210, hardmask layer 6may be formed over regions 210 and exposed portions of substrate 2between regions 210. Hardmask layer 6 adheres to regions 210 andintervening portions of substrate 2. Hardmask layer 6 may be formed frommaterials such as TEOS-type silicon dioxide, silicon nitride,polysilicon, titanium nitride, aluminum oxide (Al₂O₃), amorphous carbon,or other suitable material, depending on the material of substrate 2,first layer 4, second layer 8, and the intended etch chemistry to beused with hardmask layer 6. In an exemplary embodiment, hardmask layer 6may be formed from 300 Å thick TEOS-type silicon dioxide. Hardmask layer6 may be deposited by a process useful for precisely defining athickness thereof such as, for example, low pressure chemical vapordeposition (LPCVD) or atomic layer deposition (ALD).

Referring to FIG. 2G, following deposition of hardmask layer 6, hardmasklayer 6 and second layer 8 may be anisotropically etched to leave onlyportions of hardmask layer 6 covering the sides of regions 210 comprisedof the remaining portions of first layer 4 and second layer 8 to formspacers 28. In other words, the portion of hardmask layer 6 adhered tothe upper surface 26 of second layer 8 may be removed by etching, whichalso removes the portions of hardmask layer 6 on substrate 2 betweenregions 210. Second layer 8 may not be completely etched through as isshown in FIG. 2G, leaving a sufficient thickness of second layer 8remaining to allow spacer 28 to be of stable (constant) thickness at aheight at least coincident with the top of first layer 4.

Referring to FIG. 2H, following etching of hardmask layer 6, upperportions of spacers 28 formed from hardmask layer 6 and all of secondlayer 8 may be removed by an abrasive planarization process such as CMP.A CMP compound may be selected for the polishing slurry that will notremove first layer 4, instead stopping on upper surface 16 thereof. Inan exemplary embodiment, spacers 28 may be about 2000 Å in height. Theremaining spacers 28 formed of hardmask layer 6 exhibit a symmetric,rectangular cross-section.

Referring to FIG. 21, first layer 4 may then be removed using aselective dry or wet etching process, leaving only the spacers 28 formedof hardmask layer 6 on substrate 2 as hardmask elements. Subsequently,substrate 2 may be etched to form wells (shown by the dashed lines) orapertures having a controlled and accurate profile free from sputteringdefects using the hardmask elements comprising spacers 28 formed ofhardmask layer 6 as an etch mask. Due to the symmetrical cross-sectionof spacers 28 and the rectangular, squared-off upper portion of thecross-section, the etch rate on each side of spacers 28 is substantiallythe same, eliminating asymmetry in the pattern etched into substrate 2.Etched features in substrate 2 using hardmasks of the present inventionmay have aspect ratios of up to 5:1 or more.

In another exemplary embodiment of a method of the present invention,hardmask spacers of a symmetric geometry may be formed to various widthsaccording to the design of the desired structural features to be etchedin substrate 2. Referring to FIG. 3A, a plurality of discrete structuresin the form of regions 210 may be formed on substrate 2. Regions 210shown in FIG. 3A include first layer 4 as previously described, boundedby spacers 28 made from hardmask layer 6. Regions 210 shown in FIG. 3Amay be formed as in the previous exemplary embodiment as shown anddescribed with respect to FIGS. 2A-2H. Referring to FIG. 3B, aphotoresist 30 may be applied to upper surface 32 of first layer 4 andselectively patterned and developed using conventional photolithographytechniques to cover upper surfaces 32 of at least one region 210.

Referring to FIG. 3C, following application of resist 30, first layer 4may be removed from regions 210 that are not protected with photoresist30 using an appropriate selective dry or wet etching process. Next,photoresist 30 may then be removed using conventional techniques. Thus,subresolution hardmask elements are formed of spacers 28 where firstlayer 4 has been removed, while larger, conventionally dimensionedhardmask elements are formed from the regions 210 where first layer 4remains flanked by spacers 28, the larger hardmask elements being ofonly slightly larger dimension than that of each region 210 prior todeposition of hardmask layer 6. By intentionally undersizing a givenregion 210 intended to form a larger hardmask element to allow for theadded width provided by spacers 28, the dimensions of larger hardmaskelements may be precisely controlled. Subsequently, substrate 2 may beetched to form wells or apertures W of various widths (shown by thebroken lines) and having a controlled and accurate profile due to thesymmetric profile of the spacers 28 formed of hardmask layer 6. Thus,both conventionally dimensioned features as well as subresolution-sizedfeatures may be formed. Alternatively, by using an etchant suitable forremoval of the material of layer 4 as well as substrate 2, wells orapertures W₁ and W₂ of different depths may be formed, as depicted inFIG. 3D. As in the previous exemplary embodiment, due to the precise,squared-off cross-section of spacers 28, the etch rate on each side ofspacers 28 as well as of regions 210 having flanking spacers 28 issubstantially the same. Etched features in substrate 2 using hardmasksof the present invention may have aspect ratios of up to 5:1 or greater.

FIGS. 4A-4K illustrate yet another exemplary embodiment of a method ofthe present invention suitable for producing hardmask spacers of variouswidths yet having an accurate, symmetric, rectangular geometry.Referring to FIG. 4A, substrate 2 is provided of a suitablesemiconductor substrate such as silicon, polysilicon, or a layeredsemiconductor structure such as a silicon on insulator (SOI) structure,as exemplified by silicon on glass (SOG) and silicon on sapphire (SOS)structures. Substrate 2 may also be a glass useful for forming reticlessuch as soda-lime glass, borosilicate glass, or quartz. Then, as shownin FIG. 4B, a first layer 4 having upper surface 16 may be formed on andadhered to portion 400 of substrate 2 using techniques such as CVD, PVD,or ALD. First layer 4 may be formed from materials such as siliconnitride or other suitable material as noted above. In an exemplaryembodiment, first layer 4 may be formed from 500 Å thick siliconnitride.

Referring to FIG. 4C, following formation of first layer 4, second layer8 may be deposited on top of and adhered to the upper surface 16 offirst layer 4. Second layer 8 may be formed from TEOS-type silicondioxide or other suitable material as noted above. Second layer 8 may bedeposited using techniques such as CVD, PVD, or ALD. In an exemplaryembodiment, second layer 8 may be formed from 500 Å thick TEOS-typesilicon dioxide. Next, referring to FIG. 4D, third layer 34 having uppersurface 37 may be formed from aluminum oxide (Al₂O₃). In an exemplaryembodiment, third layer 34 may be formed from 100 Å thick Al₂O₃.

Referring to FIG. 4E, following deposition of third layer 34, portion400 may be covered by a patterned and developed photoresist 36 andanisotropically etched using conventional techniques known to those ofordinary skill in the art to form discrete structures in the form ofregions 410. Referring to FIG. 4F, regions 410 are shown after etchingand removal of photoresist 36. As shown in FIG. 4F, regions 410 mayexhibit a geometry defined by substantially vertical sidewalls 42extending from substrate 2 to upper surfaces 40.

Referring to FIG. 4G, following formation of regions 410, photoresist 38may be selectively patterned over the upper surface 40 of one or moreregions 410. The regions 410 covered and protected by photoresist 38will be referred to as regions 420.

Referring to FIG. 4H, hardmask layer 6 may be formed over substrate 2,regions 410 formed of first layer 4, second layer 8, third layer 34 and,where photoresist 38 is present, regions 420. Hardmask layer 6 coversportions of substrate 2 between regions 410 and 420. Hardmask layer 6adheres to first layer 4, second layer 8, third layer 34, photoresist 38and intervening portions of substrate 2. Hardmask layer 6 may be formedfrom materials such as TEOS-type silicon dioxide, silicon nitride,polysilicon, titanium nitride, amorphous carbon, or aluminum oxide(Al₂O₃) depending on the material of substrate 2, first layer 4, secondlayer 8, third layer 34, and the intended etch chemistry to be used withhardmask layer 6. In an exemplary embodiment, hardmask layer 6 may beformed from 300 Å thick TEOS-type silicon dioxide. Hardmask layer 6 maybe deposited by a process such as low pressure chemical vapor deposition(LPCVD) or atomic layer deposition (ALD).

Referring to FIG. 4I, following deposition of hardmask layer 6, hardmasklayer 6, third layer 34 and a portion of second layer 8 may beanisotropically etched in regions 410 where resist 38 is not present,leaving only portions of hardmask layer 6 forming spacers 28 coveringthe sides of first layer 4 and second layer 8. In the regions 410 wheresecond layer 8 is etched, it is not completely etched. Instead, asufficient thickness of second layer 8 remains after this etching stepto ensure spacers 28 are at a stable (constant) thickness at least to alevel coincident with the top of first layer 4. Furthermore, in theregions 420 where third layer 34 is protected by photoresist 38, it isnot completely removed. Instead, the remaining portion of third layer 34may be typically about 80 Å thick following etching.

Referring to FIG. 4J, following etching of hardmask layer 6, regions 410and regions 420 may be planarized using an abrasive process such as CMP.While regions 420 are slightly higher than regions 410, the CMP processis capable of simultaneously planarizing both regions due to the CMP padbeing formed of a yieldable or deformable material. A CMP compound for aslurry may be selected that will stop on first layer 4 in regions 410and stop on the remaining thickness of third layer 34 in regions 420.The remaining sidewall portions of hardmask layer 6 forming spacers 28exhibit a symmetric, rectangular profile.

Referring to FIG. 4K, after CMP, first layer 4 may be removed fromregions 410 using a selective dry or wet etching process, leaving onlythe spacers 28 formed of hardmask layer 6 on substrate 2 assubresolution hardmask elements. The material of the remaining portionof third layer 34 is resistant to the etchant used to remove first layer4. Therefore, third layer 34 protects underlying first layer 4 andsecond layer 8 in regions 420 from being removed, leaving a much largerhardmask element extending between spacers 28 bridged by the material ofthird layer 34. Subsequently, substrate 2 may be etched to form wells orapertures (shown by the dashed lines) having a controlled and accurateprofile due to the symmetric profile of spacers 28 formed of hardmasklayer 6. The etch rate of the material of substrate 2 on each side ofspacers 28 is substantially the same. Etched structural features insubstrate 2 using hardmask elements of the present invention may haveaspect ratios of up to 5:1 or greater. In regions 420 where third layer34 remains, bridging spacers 28 formed on the sides of third layer 34,second layer 8 and first layer 4, hardmask features have a slightlygreater dimension than the original photopatterned and etched regions410 prior to formation of hardmask layer 6 thereover, which dimensionmay be compensated for by slightly undersizing regions 410 to be used asregions 420. As a result, the present invention may be used tofacilitate simultaneous formation of both conventionally dimensioned andsubresolution-dimensioned features in substrate 2.

Although the foregoing description contains many specifics, these arenot to be construed as limiting the scope of the present invention, butmerely as providing certain exemplary embodiments. Similarly, otherembodiments of the invention may be devised which do not depart from thespirit or scope of the present invention. The scope of the invention is,therefore, indicated and limited only by the appended claims and theirlegal equivalents, rather than by the foregoing description. Alladditions, deletions, and modifications to the invention, as disclosedherein, which fall within the meaning and scope of the claims areencompassed by the present invention.

1. An article of manufacture comprising: a substrate; and at least onesubresolution hardmask element including a first side and a second sideconfigured to promote etching of the substrate adjacent each of thefirst and second sides at substantially the same etch rate.
 2. Thearticle of manufacture of claim 1, wherein the at least onesubresolution hardmask element includes a top surface substantiallyperpendicular to the first and second sides thereof.
 3. The article ofmanufacture of claim 1, wherein the at least one subresolution hardmaskelement exhibits a rectangular cross-section.
 4. An article ofmanufacture comprising: a substrate; and a plurality of hardmaskelements, at least some of the hardmask elements being of subresolutiondimensions and at least one of the hardmask elements being of adimension larger than subresolution.
 5. The article of manufacture ofclaim 4, wherein the at least some hardmask elements include a firstside and a second side configured to promote etching of the substrateadjacent each of the first and second sides at substantially the sameetch rate.
 6. The article of manufacture of claim 4, wherein the atleast some hardmask elements exhibit a rectangular cross-section.
 7. Thearticle of manufacture of claim 4, wherein the at least one hardmaskelements comprises at least one layer of material flanked by spacers ofsubresolution dimension.
 8. The article of manufacture of claim 7,wherein the at least one layer of material comprises a plurality oflayers.